window.addEventListener('load',function(){
    let al=this.document.querySelector('.back-center_loop_al');
    let ar=this.document.querySelector('.back-center_loop_ar');
    let loop=this.document.querySelector('.back-center');
    
    /* 鼠标经过时就显示左右箭头*/
    loop.addEventListener('mouseenter',function(){
        al.style.display='block';
        ar.style.display='block';
    })
    /* 鼠标离开 */
    loop.addEventListener('mouseleave',function(){
        al.style.display='none';
        ar.style.display='none';
    })
    
})

